docs: def correlation_matrix(df):
      '''
      Returns a pandas DataFrame with the pair-wise correlations of the columns.
  
      Args:
          df: pandas DataFrame with columns to run diagnostics on
      '''
      columns = df.columns.tolist()
      corr = pd.DataFrame(
          np.corrcoef(df, rowvar=0), columns=columns, index=columns)
      return corr